Confirmation of Laser-induced Raman scattering at Cerro Pachon

For many years now most major astronomical observatories have been using adaptive optics (AO) with the aid of optical or ultra-violet lasers to generate artificial guide stars. While the majority of laser-based AO systems in use have been used to feed infrared science cameras, there have always been scientific reasons to use these systems with optical cameras as well. One challenge in the optical region is that these lasers themselves contaminate the sky at their respective wavelengths. This contamination is an issue not only for the telescope itself but also for any other optical telescope looking at or near the laser beam. Up until recently it was thought that these lasers only contaminated the sky at their respective wavelengths. However, recent studies at the VLT have shown that there are additional, albeit much weaker, sources of contamination caused by inelastic Raman scattering of the laser light, also in the optical region. These observations were made with the VLTs MUSE spectrograph looking at the ESO 4LGSF which uses 4 Toptica Sodium Star 20/2 lasers to make a variety of different laser guide star constellations both on axis and off axis from the science camera, the total output power of the 4 lasers is on the order of 88W. At the Gemini South observatory at Cerro Pachon we propagated either a single 45W Lockheed Martin Coherent Technologies (LMCT) pulsed laser, or a single Toptica Sodium Star 20/2 continuous wave laser. In each case the beam was split into 5 beams of equal intensity and center-launched so as to always be on axis with the science camera. We used the GMOS-S spectrograph to take on axis spectra of the laser beams, looking for the same Raman lines detected at the VLT. For both lasers, we were able to detect the Raman emission lines. In this paper we will present the results from our GMOS-S spectroscopy detailing the wavelength and intensities of the Raman emission for each laser.